Title Molecular and atomic gas in planetary nebulae
Pi P. Cox
Time 73.2 hrs
Molecular and atomic gas in planetary nebulae
Authors Pierre Cox
2. Science goal(s)
In spite of the wind interaction and the onset of photo-ionization
through the post-AGB phase, a significant component of molecular gas
is found in many bone fide PNe, even in highly evolved ones such as
the Helix nebula. CO and H$_2$ have been detected in more than 50 PNe
and other molecular species have been detected in a few PNe indicating
an on-going chemistry. In most cases, the molecular gas is found
around the waist of the ionized gas in toroid-like shapes and
represent a main structural feature of the nebula and an important key
to its morphology. In addition to molecular gas, there is evidence for
neutral atomic gas in PNe, e.g., from observations of the
fine-structure lines of carbon. This gas is at the interface between
the molecular and ionized gas, and in envelopes that are essentially
completely atomic. The masses in these components are substantial but
they have not been studied in large numbers of PNe or at high angular
resolution.
The detailed structure of the molecular gas in PNe is of great
interest since it contains information on the physical processes that
produce the nebulae. The molecular gas in PNe is characterized be a
high degree of fragmentation. For instance, the envelope of the Helix
is found to be made of thousands of small (a few arcsec), dense, i.e.
10(5) cm(-3) clumps slowly evaporating in the radiation of the central
white dwarf. The origin of these tiny structures is still debated.
Another aspect of the distribution of molecular gas is its global
structure which preserves imprints of the early interaction of the
envelope with collimated, bipolar outflows or jets just after the AGB
phase.
The increase in sensitivity and angular resolution provided by ALMA
will allow us to study the molecular and atomic gas in many more
PNe. In addition, due to its possibility to measure at high
frequencies and to its flexibility in frequency set-up, ALMA will open
up the exciting possibility to observe at high spatial resolution the
warm dense gas as well as the dust continuum in planetary nebulae. The
proposed observations will provide key information to further explore
the physical and chemical conditions pertaining in the fragmented,
neutral equatorial structures which have been only recently realized
to be common and important features of planetary nebulae.
3. Number of sources: PN in southern hemisphere, including the
Helix due to its proximity (200 pc) will
be a prime target to study in detail the
properties of the cometary globules. The
PN should span the range from young PN
to fully developed such as the Helix.
Number TBD: should be of order 10
list to be defined
4. Coordinates and expected fluxes (examples with measured radio data):
4.1 Rough RA and DEC
Name RA Dec (J2000.0)
Helix nebula 22:29;38.7 -20:50:15
4.2. Moving target: yes/no (e.g. comet, planet, ...)
no
4.3. Time critical: yes/no (e.g. SN, GRB, ...)
no
5. Spatial scales:
5.1. Angular resolution (arcsec):
<0.10
5.2. Range of spatial scales/FOV (arcsec):
(optional: indicate whether single-field, small mosaic,
wide-field mosaic...)
The Helix is extended and highly fragmented. mapping of the
northern and western ridges of the nebular envelope in the
CO(2-1) transition - need of mosaicing (2 x 20 fields at 230 GHz)
5.3. Single dish total power data: yes/no
no
5.4. ACA: yes/no
no
5.5. Subarrays: yes/no
no
6. Frequencies:
6.1. Receiver band: Band 3, Band 6, Band 7, Band 9
6.2. Lines and Frequencies (GHz):
Transitions of CO, CN, HCN and HCO+
6.3. Spectral resolution (km/s):
0.1 (for the Helix nebula) to 1 km/s
6.4. Bandwidth or spectral coverage (km/s or GHz):
10 km/s 9for Helix) to about a 100-200 km/s
7. Continuum flux density:
7.1. Typical value (Jy):
(take average value of set of objects)
(optional: provide range of fluxes for set of objects)
10 mJy to 1 Jy
7.2. Required continuum rms (Jy or K):
0.01 Jy
7.3. Dynamic range within image:
>100
8. Line intensity:
8.1. Typical value (K or Jy):
For CO, typically 1 K
8.2. Required rms per channel (K or Jy):
of order 0.01 K
8.3. Spectral dynamic range:
> 100 (for brightest lines)
9. Polarization: no
10. Integration time for each observing mode/receiver setting (hr):
Per source:
10 min/Bd 3, 20 min/Bd 6 30 min/Bd 7, 9
- for 0.3 arcsec resolution and 0.2-0.3 K (1 sigma) sensitivity
Number of settings per Band will be 4.
For Helix, mosaic in CO(2-1) of two 100x100 arcsec^2 fields (north
and west).
11. Total integration time for program (hr):
10 x 4 x 10m : 6.6 h Band 3
10 x 4 x 20m: 13.3 h Band 6
10 x 4 x 30m: 20.0 h Band 7
10 x 4 x 30m: 20.0 h Band 9
2 x 20 x 1 x 20m: 13.3 h Band 6
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73.2 h total
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Review Peter Schilke: Scientifically well justified proposal, although
I don't understand how one wants to get information of the atomic gas
by observing just molecules - atomic carbon in the Japanese extension
would be interesting. I am surprised that, for the large-field
mosaicing of the Helix, neither ACA nor Single Dish data are needed -
are just the clumps interesting? The time estimate is again very
sketchy and uncertain. The need for the spectral/spatial resolution
used isn't justified.
Reply Cox: For the atomic gas, I had in mind the Japanese contribution
to study the [CI]. As for the Helix, the molecular emission is indeed
concentrated in the small (arcsecond) globules which carry all the
molecular mass. There is therefore no need for ACA nor single
dish. For the spectral and spatial resolution, the need for high
resolution is even more important than in the case of PPN, since the
lines are sub-km/s (as in the Helix) and the cometary globules have
sizes on the sub-arcsecond scale. The time estimate could be improved
indeed.
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Review v2.0:
3.4.2 Molecular and atomic gas in PN Cox 73
This project should be updated to include B8 as per the response to
referee comments included in the previous DRSP.
Would the ~600.2 GHz line of [O II] (2Do-2Do 5/2 - 3/2) be observable?